In accordance with findings presented in “The Impact of Counterfeiting on Governments and Consumers, a report commissioned by BASCAP”, Business Action to Stop Counterfeiting and Piracy, 2009, counterfeiting costs the G20 countries more that a 100 billion € per year. In the never-ending race against counterfeiters, governments and institutions have, until now, maintained a suitable lead by wielding technology to their advantage; in this respect, the currency industry is an excellent example. One of the most important efforts has been to “educate” the general public to detect false bills and/or documents. Consequently, various optical security features such as holograms for example mentioned by I. M. Lancaster and A. Mitchell in “The Growth of Optically Variable Features on Banknotes”, Proc. SPIE 5310, 34-45 (2004), Interference Security Image Structures (ISIS) mentioned by J. A. Dobrowolski, F. C. Ho, and A. Waldorf in “Research on Thin Film Anticounterfelling Coatings at the National Research Council of Canada”, Appl. Opt., 28, 2702-2717, (1989), watermarks and micro printing mentioned by R. van Renesse in “Optical Document Security”, Artech House, Delft, The Netherlands, 1998, 2nd ed. have been in circulation for many years.
Various ways are known to incorporate an ISIS into a banknote, a passport document, or a (security) plastic ID card. The ISI structure can be simply transferred and glued onto a surface, and this is suitable in many applications. The ISIS can also be weaved into the document or banknote substrate. It can also be printed or laminated between two transparent media (for example in the case of a window-type device).
Holograms and ISIS's are part of a specific category of devices termed iridescent, meaning that their appearance is dependent on the conditions under which they are observed. The color change as a function of the observation angle offered by ISIS inhibits reproduction by most reprographic techniques such as printing, scanning, etc. In “Comparative Analysis of Public Opinion Research in the U.S. and Canada”, Proc. SPIE 5310, 13-24 (2004), L. Setlakwe and L. A. DiNunzio have found that this observation angle dependent optical effect has also proven to be a highly efficient means of detection by the general public. Unfortunately, iridescent consumer products are now also readily available, and may render basic interference based devices obsolete. For this reason, various solutions have been proposed, such as combining ISIS devices with holographic elements as mentioned by P. G. Coombs, A. Argoitia, V. P. Raksha, and R. W. Phillips in “Integration of Contrasting Technologies Into Advanced Optical Security Devices”: Proc. SPIE 5310, 299-311 (2004), magnetic elements as mentioned by P. G. Coombs, V. P. Raksha and T. Markantes in “Overt and Covert Verification via Magnetic Optical Security Devices”: Proc. SPIE 4677, 182-193 (2002), etc. Another solution has been presented in co-pending US patent application publication 2008/0031508, entitled “Interference Security Image Structure”, published on Feb. 7, 2008, the specification of which is hereby incorporated by reference.
As defined by the International Commission on Illumination (CIE: “Colorimetry” International Commission on Illumination, Vienna, Austria, 2004, 3rd ed), two objects which display the same color under a specific illuminant and for a specific observer are termed metameric if their reflection or transmission spectra differ in the visible spectrum. Devices based on metamerism offer a higher performance and an extra level of security which can hence prolong the life cycle of ISIS's. In fact, by matching the color at normal incidence of an interference filter with the color of a non-iridescent material (NIM), a hidden image, which only appears as the angle of observation is increased, can be fashioned. The NIM also offers the advantage of serving as a color reference making detection much easier including for observers with color vision deficiencies.